This invention relates to a micro-displacement detector, and more particularly to the structure of its sensor.
A variety of detectors adapted to mechanically detect a microdisplacement have been proposed in the art. One example of a conventional microdisplacement detector is arranged principally as shown in FIG. 1. In that detector, stationary electrodes E.sub.11 and E.sub.12 are on both sides of a movable electrode E.sub.V, connected to a feeler, to form a pair of differential capacitors C.sub.1 and C.sub.2, respectively, thus forming a sensor. This sensor is coupled to a pair of oscillators OSC.sub.1 and OSC.sub.2 which are combined with crystal oscillator units Xtal.sub.1 and Xtal.sub.2, respectively, so that outputs of frequencies f.sub.1 and f.sub.2 corresonding to the capacitance variations of the differential capacitors are applied to a mixer MIX. In the mixer MIX, the difference frequency (f.sub.1 -f.sub.2) between these frequency outputs is obtained. The difference frequency is delivered as an output Fd. The output Fd is subjected to frequency-to-voltage conversion so as to be displayed on a meter for instance.
The variations of the output frequencies f.sub.1 and f.sub.2 of the oscillators OSC.sub.1 and OSC.sub.2 can be represented substantially as hyperbolic functions as indicated in the parts (a) and (b) of FIG. 2, respectively. These oscillator outputs f.sub.1 and f.sub.2 are applied to the mixer MIX, where the difference frequency is obtained as was described above. The difference frequency thus obtained corresponds to the shaded portion in the part (a) of FIG. 3, and can be expressed as a distance vs. frequency characteristic curve as shown in the part (b) of FIG. 3.
The characteristic curve in the part (b) of FIG. 3 is substantially S-shaped as a whole; however, it has a substantially linear part K extending on both sides of a central point P. Within the range of the linear part K, the frequency of the output Fd responds substantially linearly to the movement distance d of the feeler, and accordingly the displacement can be measured with high accuracy.
However, the linear part corresponds to only about 30% of the range of movement of the feeler. Therefore, if it is required to perform the measurement with high accuracy, the measurement is limited to a relatively small range. If, on the contrary, it is required to measure over a wide range, the accuracy must be decreased.